(a) Field of the Invention:
The present invention relates to an ultrasonic method for measurement of the size of any flaw possibly existent in various kinds of solid mass.
The term "solid mass" herein used refers to the parts or members of whatever the shape, dimensions or surface roughness, constituting, for example, an electrical, mechanical, chemical equipment or installation, etc. in various industrial fields. Further, the "solid masses" to which the present invention is applicable include a mass of a metal as well as a mass of a non-metal such as glass, ceramic, concrete, synthetic resin, rubber, wood, and the like through which the ultrasound wave can be propagated.
Also the size of a flaw possibly existent within a solid mass, referred to herein, points to that of a flaw of whatever the position, shape and kind found within the solid mass.
(b) Related Art Statement:
In the field of technique to which the present invention is applicable, it is very important and necessary to study, from the viewpoints of the strength and life of a piece of equipment or installation, the parts or members thereof for any flaw within it, and know, if any flaw is found within the part or member, what the flaw really is by measuring the position, shape and kind thereof and specifically the size thereof as well, thereby analyzing accurately the extent of adverse affect of the flaw to the part or member.
Such study is indispensable for essential parts or members. For this purpose, various techniques for radiographic tests utilizing radiations such as X-rays or gamma-rays as well as ultrasonic techniques have so far been employed for detection of any flaw.
By the technique for radiographic tests, an object is studied and analyzed for any internal flaw by observing an image produced with the radiation having been transmitted through the object, namely, by observing the change in the intensity of transmitted radiation which changes with the object shape, dimensions, material and the existence of any internal flaw. Therefore, the test results greatly depend upon the quality of the radiograph thus obtained. Since it is necessary to select a radiographic film of which the quality is suitable for radiography of an object to be tested and to do a series of photographic steps from radiography up to development, so the technique for the radiographic test is not simple and cannot be done easily and directly Also in some cases, if the film sensitivity and resolution are insufficient or even if they are not insufficient, the size of flaw cannot be measured and any flaw cannot even be detected; namely, in many cases, the technique for radiographic tests does not permit detection of what the flaw really is.
Also the ultrasonic pulse-echo techniques for detection of any internal flaw have been used from a long time. The typical and general one of such techniques is the direct-contact vertical flaw detection technique by which a vertical probe is applied in direct contact with the object (which will be referred to as "vertical flaw detection technique" hereinafter). Among the techniques of measuring the flaw size by the vertical flaw detection, three kinds of techniques are prevailing at present, including the utilization of (a) pulse-echo height; (b) probe directivity, and (c) ratio between F (flaw echo) and B (bottom echo).
Concerning the above-mentioned technique (a), the height of the echo appearing on the screen of an ultrasonic pulse echo flaw detector of A-scope display type (will be referred to as "ultrasonic flaw detector" hereinafter) depends upon the the roughness of the searching surface of an object under test, namely, the roughness of a surface upon which the ultrasonic wave is incident, attenuation of ultrasound being propagated, distance from the above-mentioned searching surface to a flaw possibly existent within the object and the size of the flaw when the total gain of the ultrasonic flaw detector is kept constant. Thus, the technique (a) is such that the object is compared with a comparison test piece prepared as reference test piece based on a known flaw size and the size of any flaw within the object is estimated through a sound field correction and a correction taking the shape or the like of the flaw in consideration. However, this technique is not highly reproducible as to the correction of sound field, flaw shape, and the like as the flaw size is larger, and also it is yet unclear in many points. So this technique is limited to the size measurement of small flaws of less than several millimeters. Also it is low in accuracy of measurement. Therefore, this technique cannot be widely adopted in measurement of flaw size.
The aforementioned technique (b) utilizes the directivity of the probe. Based on the fact that when the probe is displaced to a position where the center beam is off any flaw of a relatively large size, for example, a size larger than the transducer in the probe, the appearing flaw echo will be small, this technique makes it possible to know the flaw size from the range in which no flaw echo appears. However, this technique cannot provide accurate measurement of flaw size because of the interaction among various factors such as the uncertain directivity of reflection, requirement for sufficient distance of flaw position from the searching surface, attenuation of ultrasound, nonlinear beam propagation, etc.
The technique (c) mentioned above further includes two techniques: F/BG method and F/BF method. The former method F/BG utilizes the ratio between the height of the maximum echo from a flaw 500 (will be referred to as "F echo" hereinafter) to that of the echo of a sound pressure B from the bottom surface 100B of a whole region in which there is not the flaw 500 (which will be referred to as "BG echo" hereinafter) as shown in FIG. 10. As shown in FIG. 11, the F/BF method utilizes the ratio of the height of the F echo to that of the F echo of a sound pressure F from the flaw 500 (which will be referred to as "BF echo" hereinafter).
In the F/BG method, when the shape and the surface roughness of the bottom surface 100B of an object 100 which provides the BG echo are similar to those of the standard or comparison test piece and the flaw size is rather small as compared with the transducer size, the flaw size is quantitatively evaluated to some extent with an expression, for example, "it is approximate to STB-G, V15-4" or "it is on the order of 6 mm as converted into a circular flat flaw using the AVG diagram.
On the other hand, since the F echo detecting position is the same as the BF echo detecting position, the F/BF method is not so much affected by the shape and roughness of the searching surface 100S and the accuracy of flaw detection does not greatly depend upon such factors. Furthermore, this method is practically advantageous in that the reduction of the BF echo height due to the existence of a flaw 500, namely, the shadow effect of the flaw 500, can also be evaluated. Since this shadow effect occurs due only to the size of the flaw 500, not to the shape thereof, so any large flaw will lead to the reduction of BF echo height. Therefore, a flaw of a large size (but smaller than the size of the transducer) can be evaluated with a higher accuracy by the F/BF method through the evaluation of the F echo. However, both these F/BG and F/BF methods are not advantageous and have the problems as will be described:
(1) First, since the ratio between F and BG echoes, or the ratio between the F and BF echoes, is taken as index of evaluation, the reflectance of the bottom surface which is likely to be affected by the shape, inclination and roughness of an object under inspection should be constant.
(2) For the BG and BF echoes, the bottom surface 100B must be smooth and have an effective area of reflection, and any object of a shape and size that could not provide the BG and BF echoes cannot be measured by these methods.
(3) In case an object of which the searching surface 100S is rough such as cast surface, shot-blasted surface, etc. is measured, it is necessary to use an acoustic contact medium between the searching surface 100S and probe 400. However, even if a contact medium suitable for the shape and inclination of the searching surface 100S is selected for this purpose, air or bubble is likely to exist in the medium and degrade the ultrasound transfer characteristic of the medium, which will cause the echo height to vary. Thus, it is not possible to measure with a high accuracy.
(4) These methods need a searching surface 100S of an area which enables the probe 400 to be in contact therewith. Any object having only a narrow surface on which the probe 400 cannot be applied cannot be measured for any flaw by these methods.
(5) In case of the F/BF method, there is no correlation between the F echo height and the size of a flaw which is larger than that of the transducer, and the ultrasound will not arrive at the bottom surface 100B of the object in which the flaw exists, so that no BF echo can be obtained. So in such case, no flaw size measurement can be done by this method.
(6) In both these methods, the equation for determining the flaw size is a quartic one of which the solution takes much time.
The methods of flow detection that overcome these problems to some extent include a water or oil immersion method (will be referred to as "liquid immersion method" hereinafter). In this method, an entire object is immersed in a liquid or only the liquid is locally filled only between the probe and object, and ultrasound is transmitted through the liquid toward the searching surface from the probe located at a position of some distance from the searching surface of the object. However, this method can generally solve only the above problems (3) "the accuracy of flaw size measurement is affected by the roughness of the searching surface" and (4) "a searching surface of such a sufficient area that the probe can be applied to the object", but not the practical problems in the flaw size measurement which have been a large difficulty in the measurement of flaw size.
As has been described in the foregoing, a large quantity of objects cannot be measured directly along the production line, in other words, they cannot be measured in a short time, with a high accuracy and quantitatively, by these conventional methods.
Also, when a flaw detection is done by scanning with the probe being moved as kept applied on the surface of the object, the sound pressure of the reflected echo from a flaw varies depending upon the contact pressure of the probe and the inclination of the probe with respect to the object. The method of compensating this gain variation is disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 58-68663. This method utilizes the fact that the sound pressure of the reflective wave from the surface of the object in proportion to the variation in sound pressure of the flaw-reflected wave depending upon the condition of contact of the probe on the object. The above-mentioned Unexamined Patent Publication discloses a concept that the value of the ratio h.sub.S/F between the echoes S and F is constant independently of the conditions of ultrasound incidence from the probe. In this case, even if the amplitude of the echo from any flaw is only compensated, the size of the flaw cannot be measured. It is necessary to take in consider the relation between the size of the flaw and the location of the flaw.